I just got the E12 and I hear a constant hiss on mine using the Westone UM Pro 30(f.k.a. UM3X) unless I turn the volume up past 9 o'clock. But I would still hear it at a quiet environment and the music gets quiet as some of mine does. I didn't notice a hiss while I was auditioning it against the C5 in store. The two practically sounded the same at the lowest sound level.
BTW is it normal for the crossfeed to lower the power output when turned on? I was playing with it and the volume drops when I turn it on. I have it on at the moment as it give me more variability with the volume control. Does this do anything bad to my music?